Movement of solutes associated with intermittent soil-water flow .II. Nitrogen and cations

Abstract

The movement of applied potassium and nitrogen through columns of repacked silt loam was examined in the laboratory. The nutrients were applied as 5 mm of a dilute solution of potassium bromide and urea, left for 3 or 10 days, and then leached further down into the soil. Twelve days after the start of the experiment, the columns were sectioned to allow the distribution of applied and resident nutrients to be assessed. Most of the urea nitrogen was transformed to nitrate over the 12 days. The timing of water application relative to nitrification was important in determining the amount of nitrogen leached. The soil solution anion concentration determined the amount of cation leaching. Although potassium was the cation added, calcium and magnesium were the dominant cations accompanying the nitrate downwards.A mechanistic model was constructed, driven by the water movement, and with the solute dispersion, diffusion and adsorption parameters derived from the literature. It assumed the Gapon relationship for cation exchange, and that hydrogen ion production reduced the effective cation exchange capacity. This model was able to simulate closely the experimental data.